In the existing information recording devices, high density of information recording is realized by optical recording as represented by a magneto-optical recording scheme and a phase change recording scheme. For example, in the magneto-optical recording scheme, laser light is applied onto a recording medium having a magnetic film formed on a surface thereof. The orientation of magnetization is controlled by the synergism of reduction of coercive force due to temperature rise at a spot of applied laser light and application of external magnetic field, thereby enabling binary recording.
In the magneto-optical recording scheme, recorded information can be reproduced by illuminating, to a recording medium, laser light weaker in intensity than that in recording and then specifying an orientation of magnetization due to a state of polarization of a reflection or transmission light thereof.
Meanwhile, in the phase change recording scheme, laser light is applied to a recording medium formed with a phase change film on a surface thereof. The temperature caused at a spot of the illuminated laser light is varied by varying the intensity of laser light to control in crystallizing and amorphosizing of the phase change film, thus enabling binary recording.
In the phase change recording scheme, recorded information can be reproduced by illumination with laser light weaker in intensity than that in recording to the recording medium and distinguishing between a crystalline phase and an amorphous phase due to an intensity of reflection thereof.
The above-explained magneto-optical recording scheme and phase change recording scheme both realize high-density information recording and reproducing by the microscopic spot of laser light. Consequently, the information recording density on a recording medium is limited to by a spot diameter obtained by focusing laser light. Accordingly, in the conventional information recording apparatuses employing the magneto-optical recording scheme and the phase change recording medium, because a spot obtained by focusing laser light is utilized as propagation light, the spot diameter could not have been decreased smaller than a diffraction limit of laser light, i.e. a half of a laser light wavelength.
Under the circumstances, there is a proposal of an information recording method/apparatus for an optical memory in which laser light to be turned to propagation light is applied toward a microscopic aperture having a diameter less than a wavelength of applied laser light, e.g. one-tenth of the wavelength, to utilize near-field light produced at the microscopic aperture (including both Evanescent field and far field). In this information recording method, a mechanism of achieving information recording to a recording medium is basically the same as the near-field producing system of an information reproducing method/apparatus for reproducing recorded information on the recording medium by utilization of near-field light. That is, the information reproducing method/apparatus for an optical memory utilizing near-field light can be utilized at the same time as an information recording method/apparatus.
Conventionally, there have been, as apparatuses utilizing near-field light, near-field microscopes using a probe having a microscopic aperture as mentioned above, utilized in observing optical characteristics on a microscopic region of a sample. As one of the near-field light utilizing schemes in the near-field microscopes, there is a scheme that a microscopic aperture of a probe and a sample surface is brought into proximity in distance to nearly a diameter of the probe's microscopic aperture so that near-field can be produced at the microscopic aperture by introducing propagation light through the probe and toward the probe's microscopic aperture. In this case, the produced near-field light interacts with the sample surface to cause scattering light to be detected involving an intensity or phase reflecting a microscopic structure on the sample surface by a scattered light detector system. This achieves an optical image observation with a resolution that has never been realized in the conventional optical microscopes. The optical memory information recording method utilizing near-field light as above utilizes an observation method for the near-field microscope.
Accordingly, utilizing near-field light makes it possible to record on a microscopic information recording unit surpassing a recording density on the conventional information recording medium and to reproduce from the information recording medium thus recorded. Furthermore, as disclosed in Japanese Patent Laid-open No. 98885/1995 and Japanese Patent Laid-open No. 272279/1995, in information reproducing, the selection of a probe shape having a microscopic aperture allows for selection of information unit in reproducing. Thus, there are proposals to achieve an increase of density in forms not existing in the conventional information recording media.
As discussed above, the information recorded by the magneto-optical recording scheme is due to determination of a light polarization state of a reflected or transmitted portion of the applied light and requires the device to pass the reflection or transmission light to a photodetector. The loss of light in that case is large. Near-field light in nature possesses an extremely low intensity. It is therefore difficult to employ a magneto-optical recording scheme in an optical memory information reproducing method utilizing near-field light. At the same time, also difficult is its adoption as an optical memory information recording method.
Meanwhile, where a phase change recording method is employed in an optical memory information recording/reproducing method utilizing near-field light explained above, information recording must be made by a heat mode in which laser light energy is utilized by being transformed into thermal energy. However, because the near-field light caused at a microscopic aperture is very weak in energy, it is difficult to realize information recording by a phase change recording scheme. Even where a sufficiently high intensity of laser light is introduced to a microscopic aperture, the microscopic aperture itself gives off heat, resulting in possible damage to the recording medium or the probe tip having a microscopic aperture or adverse effect upon a control system thereof.
Also, when a probe having a microscopic aperture as above is employed as an optical memory head, the access of the probe to a distance for utilizing near-field light on a recording medium usually utilize cantilever control and detection technology for the atomic force microscope (AFM). However, in the AFM technology utilization in the near-field microscope, the transfer of heat energy from the cantilever to a sample is not considered. Due to this, various problems arise in employing a magneto-optical recording scheme or a phase change recording scheme. For example, the near-field microscopes often use a cantilever formed by an optical fiber having a microscopic aperture and propagating light through the microscopic aperture. The cantilever type optical fiber has a spring constant having a value greater than that of a silicon micro-cantilever used in the AFM. In contact control to detect a repelling force where the cantilever is contacted with a sample, there is high possibility of damaging the cantilever itself or a sample surface.
Meanwhile, in non-contact control that a sample-to-cantilever distance is increased as compared to that of contact control to microscopically vibrate the cantilever and detect modulation due to an attractive force acting between the cantilever and the sample surface, and in dynamic control that a cantilever is vibrated and the cantilever is brought into contact with a sample surface to acquire surface information, the heat transfer to the recording medium through near-field light is not steadily made. Thus, the temperature as a recording condition is impossible to reach.